For purposes of discussion herein, the invention is described in relation to its use or incorporation in a digital cordless telephone. However, it should be understood that the invention is not so limited, but will find use in many other applications using block coded data.
Presently, the design of cordless telephone systems is based primarily upon techniques for generating a speech representative signal using analog techniques and for transmitting the analog formed signal in accordance with known radio transmission techniques. It will be appreciated that analog techniques for generating speech representative signals are susceptible to interference and noise. Next generation cordless telephones will undoubtedly incorporate digital techniques for generating the speech representative signal, which digital signal would then be converted into analog form for transmission. Such next generation phones have been referred to as digital cordless telephones (DCT) or Personal Hand-Phones (PHP).
Since digital techniques effectively reduce speech representative signals to a series of numbers, the generated and reproduced speech signals would be more reliable, i.e., have less interference and noise, than the analog based speech signals produced in presently available cordless telephone systems. Indeed, digital techniques for generating speech or voice representative signals are now known for use in relation to cord-based telephone systems.
As used herein, cordless telephone or digital cordless telephone (DCT) refers to those systems intended for home, public or office use. Such systems typically include a battery powered portable station (handset) and a base station, with the base station being connected into the public telecommunication network.
Transmission standards or specifications have already been developed in both Japan and Europe for use in designing DCT systems. Each of the transmission standards are based on the use of a time division duplex (TDD) format, employing time division duplex for two-way communication. For purposes of illustration herein, the Japanese standards will be emphasized. However, it is noted that the invention will have utility with any transmission standard and is therefore not limited to use solely with the Japanese standard.
The Japanese DCT transmission standard specifies the use of a plurality of individual carrier signals having a frequency separation of 300 kHz within an overall system bandwidth of about 23 MHz between approximately 1,895 MHz to 1,918 MHz. Each carrier signal should support four channels in a TDMA format employing time division duplex for two-way communication. In particular, for each frame of time (5 ms) there are four transmit time slots (one for each channel) and four receive time slots (one for each channel). Each slot is approximately 625 .mu.s long with a guard time of approximately 30 .mu.s provided within each slot.
Under the Japanese standard, speech, representative signals are to be generated using a known digital technique, namely, the adaptive pulse code modulation (ADPCM) technique. The ADPCM signal is thereafter used to generate a digital modulated signal. The modulation scheme specified in the Japanese standard is the differential .pi./4-QPSK (.pi./4-quadrature phase shift keying) scheme with square root raised cosine filtering. It will be appreciated that such a scheme permits the transmission of digital data (1s and 0s) using a minimum number of bits. Digital data generated by this scheme is to be transmitted at a rate of 384 kHz which, in view of the modulation scheme, corresponds to a symbol transmission rate of 192 kHz. For a more detailed explanation of the differential .pi./4-QPSK modulation scheme, reference is made to application Ser. No. 999,210, filed Dec. 31, 1992 now U.S. Pat. No. 5,376,894.
By contrast, the European DCT system specifies a series of carriers spaced 1.728 MHz apart within an overall bandwidth of approximately 17.28 MHz. Each carrier is to support twelve full duplex channels, i.e., 12 slots for transmission and 12 slots for reception.
Unfortunately, the specification of particular transmission standards is only the beginning. Having established the particular parameters in which digital cordless telephone operation can occur, several technical problems arise in relation to the design and selection of components to be used for generation, transmission and reception of speech representative signals. These problems will require resolution in order to achieve the development of digital telephone equipment capable of operation within the parameters of such standards.
One such problem results from the requirement specified in the Japanese and European systems for error detection and correction (EDC). Although EDC is well known, the speed or time restrictions placed on EDC by the DCT and DECT specifications creates unique problems.
Several error detection and correction (EDC) schemes have been devised, particularly for determining whether data retrieved from a memory is identical to the data originally stored in memory and for correcting the retrieved data if it is not identical to the originally stored data. One common technique for assuring data integrity is the use of error correcting codes.
Generally, before data is written into a memory, it is passed through a logic network where the individual bits of a data word are combined in some predetermined manner to produce a series of check bits. The check bits are stored in memory in association with the data bits. When data is read or retrieved from memory, it is passed through the same logic network and new check bits are generated. The new check bits are then compared to the previously stored check bits. If an error in reading data has occurred the new check bits will not match the original check bits. The result of comparing new with original check bits is referred to as the syndrome. If sufficient check bits have been generated, then, it is possible to correct the data. The particular error correcting code chosen will determine: the type or design of the logic network utilized; the way in which data is applied to that network; and the extent to which corrections can be made.
It will be appreciated that for digital cordless telephones, voice data will not be stored or retrieved from memory, but rather, voice data will be either transmitted or received. Consequently, if error detection and correction is to occur, check bits associated with such voice data will have to be generated and transmitted with the voice data. Similarly, received voice data will have previously determined check bits concurrently received.
In the Japanese DCT system, a 16-bit Cyclic Redundancy Code (CRC) is specified. To this end, the Japanese DCT standard for transmission of data requires data to be transmitted in a particular format. Each transmission frame, i.e., each transmit slot described above, will first include channel indication and control data fields, followed by the actual voice or information data field, followed by a 16-bit CRC field.
Although, CRC has been used in the past for error detection, it is desirable to use CRC in cordless telephones for error correction as well.
Accordingly, a need exists for a digital cordless telephone system that utilizes a block code for error detection and correction. A particular need exists for a digital cordless telephone that utilizes CRC for both error detection and correction.